Abstract
In this study, the acicular ferrite transformation behavior of a Ti–Ca deoxidized low carbon steel was studied using a high-temperature laser scanning confocal microscopy (HT-LSCM). The in situ observation of the transformation behavior on the sample surface with different cooling rates was achieved by HT-LSCM. The microstructure between the surface and interior of the HT-LSCM sample was compared. The results showed that Ti–Ca oxide particles were effective sites for acicular ferrite (AF) nucleation. The start transformation temperature at grain boundaries and intragranular particles decreased with an increase in cooling rate, but the AF nucleation rate increased and the surface microstructure was more interlocked. The sample surface microstructure obtained at 3 °C/s was dominated by ferrite side plates, while the ferrite nucleating sites transferred from grain boundaries to intragranular particles when the cooling rate was 15 °C/s. Moreover, it was interesting that the microstructure and microhardness of the sample surface and interior were different. The AF dominating microstructure, obtained in the sample interior, was much finer than the sample surface, and the microhardness of the sample surface was much lower than the sample interior. The combined factors led to a coarse size of AF on the sample surface. AF formed at a higher temperature resulted in the coarse size. The available particles for AF nucleation on the sample surface were quite limited, such that hard impingement between AF plates was much weaker than that in the sample interior. In addition, the transformation stress in austenite on the sample surface could be largely released, which contributed to a coarser AF plate size. The coarse grain size, low dislocation concentration and low carbon content led to lower hardness on the sample surface.
Highlights
The acicular ferrite formed in steel can be used in the technology called oxides metallurgy, where the acicular ferrite can nucleate at the surface of oxide inclusion during the transformation from austenite to ferrite
Hanamura et al [12] used this method to observe the nucleation of acicular ferrite (AF) around inclusions and confirmed the thermodynamic calculation results by in situ observation in 1999
With a coarse grain size, a lower dislocation concentration under high transformation temperature, and carbon diffusion away from the surface ferrite into the sample interior, the surface microstructure hardness was much lower than the whole AF microstructure in the same sample
Summary
The acicular ferrite formed in steel can be used in the technology called oxides metallurgy, where the acicular ferrite can nucleate at the surface of oxide inclusion during the transformation from austenite to ferrite. It is well known that the formation of acicular ferrite is strictly influenced by inclusion type, prior austenite grain size and cooling rate [1,2]. Mu et al discussed the effects of the cooling rate, grain size, and inclusion composition on the ferrite fraction and phase-transition temperature [8,19]. Wang et al studied the nucleation and growth of AF at different cooling rates in Ti-Ca-Zr deoxidized low-carbon steel [20]. In order to clarify the difference between the in situ observation of the sample surface and the phase transition in the sample interior, and to better understand the AF phase transformation mechanism, the microstructure between the surface and interior of the HT-LSCM sample, in a low C–Mn steel with effective inclusions at different cooling rates, were characterized and analyzed
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